ES-4024A User’s Guide

CHAPTER 28

OSPF

This chapter describes the OSPF (Open Shortest Path First) routing protocol and shows you how to configure OSPF on the ES-4024A.

28.1 Overview

OSPF (Open Shortest Path First) is a link-state protocol designed to distribute routing information within an autonomous system (AS). An autonomous system is a collection of networks using a common routing protocol to exchange routing information.

OSPF offers some advantages over traditional vector-space routing protocols (such as RIP). The following table summarizes some of the major differences between OSPF and RIP.

Table 61 OSPF vs. RIP

 

OSPF

RIP

 

 

 

Network Size

Large

Small (with up to 15 routers)

 

 

 

Metrics

Bandwidth, hop count, throughput, round

Hop count

 

trip time and reliability.

 

Convergence

Fast

Slow

 

 

 

28.1.1 OSPF Autonomous Systems and Areas

An OSPF autonomous system can be divided into logical areas. Each area represents a group of adjacent networks. All areas are connected to a backbone (also known as area 0). The backbone is the transit area to route packets between two areas. A stub area, at the edge of an AS, is not a transit area since there is only one connection to the stub area.

The following table describes the four classes of OSPF routers.

Table 62 OSPF: Router Types

TYPE

DESCRIPTION

 

 

Internal Router (IR)

An Internal or intra-area router is a router in an area.

 

 

Area Border Router (ABR)

An Area Border Router connects two or more areas.

 

 

Backbone Router (BR)

A backbone router has an interface to the backbone.

 

 

AS Boundary Router

An AS boundary router exchanges routing information with routers in other

 

ASes.

Chapter 28 OSPF

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